Effects of TiO2 surface fluorination on photocatalytic degradation of methylene blue and humic acid

Photocatalytic degradation (PCD) reactions of cationic methylene blue (MB) and anionic humic acid (HA) were studied in naked TiO2 and fluorinated TiO2 (F-TiO2) suspensions in order to investigate how the modification of the TiO2 surface functional group influenced PCD reactions. Adsorption behaviors of MB and HA in the naked TiO2 followed a typical pH-dependent electrostatic interaction mechanism. On the other hand, those in the F-TiO2 were markedly changed and even showed a reversed dependence in specific pH ranges due to surface fluoride interrupting the interaction of substrates and surface titanol groups. PCD rates of MB (k (MB)) and its N-demethylation (Delta lambda (max)) were significantly increased by surface fluorination below circum-neutral pH range, in particular, by a factor of 12 and 54 at pH 2, respectively. In the case of HA, the fluorination had an insignificant effect on its degradation rate but appeared to change its degradation behavior. It has been suggested that, although the primary effect of fluorination enhances the photocatalytic production of hydroxyl radicals, the change in electrostatic interaction with substrates could affect PCD as well.